No sound system

Google is your best friend :

http://x-driver-download.blogspot.fr/2012/05/sony-vaio-pcg-5c9p.html

Get onlineand got to www.google.com In the google search box type: "SonySupport" (without the quotes and put your manufacturer name before Support). Look for downloads and drivers. Enter your model number and operatingsystem when requested. Download and install the Audio drivers.

You can restore the computer to previous date where the computer was working fine.

You can download the audio and device drivers from the Sony WEB site.
Select your model notebook and the Windows drivers you require. Please click on this link to the Sony support WEB site :-
http://esupport.sony.com/US/perl/select-system.pl?PRODTYPE=1,24,72&DIRECTOR=DRIVER

Sony Vaio PCG and some Vaio VGN Series use the last 7 digits of the laptop serial number to calculate the bios master password. The serial number for a Sony Vaio is often located on the bottom of the laptop on a white sticker. The serial number may also be found by removing the battery and checking in the battery compartment. The link to unlockbiosrecovery.com below works on the following models:
PCG-C, PCG-F, PCG-GR, PCG-GRS, PCG-GRT, PCG-GRV, PCG-GRX, PCG-GRZ, PCG-N, PCG-NV205, PCG-QR, PCG-R, PCG-SR, PCG-SRX, PCG-TR, PCG-V, PCG-VX, PCG-X, PCG-Z, VGN-B, VGN-FS, VGN-S, VGN-T, VGN-TX1, VGN-X
I use http://www.unlockbiosrecovery.com/Unlock_Sony_Bios.php
For $20 they use the last 7 digits of the laptop serial number to calculate the bios master password and then email it to you in an hour. They guarantee it to work and you can call them if you have a question.

Hello,

enter your model number on the page : http://esupport.sony.com/

Reboot the laptop. While rebooting keep pressing F8. This should get you into the menu then choose "repair my computer". If you want you can restore it to factory as if the laptop was new again. Back up files you want to keep before you do this as the only thing that will be left is the operating system.

Device Type: Sound Card
Manufacturer: Sony Electronics
Model: Sony VAIO VGN-B55G or PCG-5B5P
Operating System(s): Windows® XP
ADI SoundMAX® Audio Device Driver @ http://esupport.sony.com/US/perl/swu-download.pl?mdl=VGNS170&upd_id=1571&os_id=7

Hope it helps.

http://esupport.sony.com/US/perl/swu-list.pl?mdl=VGN-FS980
ALL Drivers are available at this website above.

http://esupport.sony.com/US/perl/swu-list.pl?mdl=VGNFS980
Go the above link select the operating system and then give AUDIO in file category opiton you will get the required driver.